Conventionally, switching power supply apparatuses including semiconductor devices have widely been used as power supply apparatuses of electronic equipment such as home appliances and office equipment, for example, to improve the power conversion efficiency. The semiconductor devices include a semiconductor, that is, a switching element such as a transistor, and control an output voltage through a switching operation performed by the semiconductor.
In particular, reduction in power consumption of the electronic equipment on standby has received attention to prevent global warming, and switching power supply apparatuses having lower standby power consumption have grown in demand.
In recent years, a system of an AC adapter or a charger has been proposed which almost eliminates the power consumption, by reducing the output voltage when the switching power supply apparatus does not have to supply power, for example, when the system is disconnected from a loading apparatus (see Patent Literature (PTL) 1).
FIG. 15 is a circuit diagram illustrating a configuration example of a conventional switching power supply apparatus and a conventional semiconductor device. Specifically, FIG. 15 illustrates the configuration example of the conventional switching power supply apparatus that reduces an output voltage Vout on standby. The operations on standby will be simply described hereinafter.
A standby control circuit 791 included in a loading apparatus 790 at the secondary monitors a loading state. When shifting to the standby mode, the standby control circuit 791 causes a standby mode control circuit 690 to pass a current to a photocoupler 12. When the photocoupler 12 transmits a standby signal to the primary, the current flows through a resistor 17. Then, the voltage of an STB terminal of a switching control circuit 190 increases. When a standby mode detector 193 detects that the voltage of the STB terminal is higher than that of a comparison reference voltage source 194, a pulse width control circuit 191 shifts to the standby mode for suppressing the switching operation of a switching element 7 through a driving circuit 192 and a resistor 6 and decreasing the output voltage Vout of the switching power supply apparatus. When the standby mode is restored to the normal operation, the switching power supply apparatus cancels the standby mode and outputs a specified output voltage Vout by stopping the operation of the photocoupler 12 and decreasing the voltage of the STB terminal of the switching control circuit 190.
This system can reduce unnecessary power wasted for obtaining a voltage on standby more than the power for generating the specified output voltage Vout regardless of the operation state of the loading apparatus.